Thermography is an image-forming process including a heating step and hence includes photothermography in which the image-forming process includes image-wise exposure and direct thermal processes in which the image-forming process includes an image-wise heating step. In direct thermal printing a visible image pattern is produced by image-wise heating of a recording material.
U.S. Pat. No. 3,080,254 discloses a heat-sensitive chemically reactive copy-sheet suitable for the preparation from differentially radiation-absorptive graphic originals of thermographic reproductions having dark-colored image areas of pleasing appearance, said copy-sheet comprising a thin flexible carrier web-coated with a visibly heat-sensitive coating comprising (1) a film-forming binder, (2) a noble metal salt of an organic acid, and (3) a cyclic organic reducing agent for the noble metal ions, having an active hydrogen atom attached to an atom which is selected from the class of oxygen, nitrogen and carbon atoms and is directly attached to an atom of the cyclic ring, and additionally including (4) a significant small proportion, sufficient to cause observable darkening of the thermographic image, of a heterocyclic organic toning agent containing at least two hetero atoms in the heterocyclic ring, of which at least one is a nitrogen atom, such as phthalazinone, barbituric acid, 2-benzoxazolethiol and 1-acetyl-2-thiohydantoin.
Thermographic and photothermographic materials with prior art toning agents exhibit poor storage properties, as is the case with e.g. phthalazinone, and/or an image colour which has an insufficiently neutral tone for black and white images, as is the case with e.g. succinimide, phthalimide, phthalic acid and phthalazine. The use of 3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine as a toning agent in thermographic materials, as disclosed in U.S. Pat. No. 3,951,660, represented an improvement in the neutrality of the image tone, whether substituted, as disclosed in U.S. Pat. No. 3,885,967 and U.S. Pat. No. 3,951,660, or unsubstituted, as disclosed in U.S. Pat. No. 3,951,660. However, such toning agents are insufficiently soluble in ecologically acceptable coating solvents and diffuse through the thermographic materials to the thermal head resulting in cloudiness in the imaging material, deposits on the surface of the thermographic material and, in the case of substantially light-insensitive thermographic materials in thermal head printers, image degradation due to thermal head contamination. There is therefore a need for alternative toning agents in thermographic materials, which do not have an adverse effect on the image tone.